Magnesium alloys have attracted attention as alloys for practical use because they have a small weight and excellent electromagnetic shielding properties, machinability, and recyclability, but they are known to have resistance to plastic processing at room temperature. For this reason, the conventional magnesium alloys that have been used, for example, for press forming had to be formed at an elected temperature (150 to 350° C.). From the standpoint of operability, safety, and cost, it was also desired that materials with formability at room temperature be developed.
Mg is considered to have poor formability because it has a hexagonal closest packed crystal structure (h. c. p.) with few slip planes during plastic deformation. Accordingly, attempts have been made to increase formability by changing the crystal structure (increasing the number of slip planes) by means of adding various alloying elements to Mg.
Among the alloys thus obtained, an Mg—Li eutectic alloy is an alloy in which a P-phase, which has a body centered cubic crystal structure (b. c. c.) with a solid solution of Li in Mg is precipitated by adding Li in an amount of no less than 6%, and formability is thereby increased. Such Mg—Li eutectic alloys can be subjected to forming at room temperature and this specific feature of the alloys offers strong possibility for new processing methods.
However, though such Mg-Li eutectic alloys have excellent room-temperature formability, the drawback thereof is that the increase in formability is accompanied by the decrease in tensile strength and that the addition of active elements Li decreases corrosion resistance. When a large amount of Al, Zn, or the like is added to improve the tensile strength and corrosion resistance, the room-temperature formability, which is a specific feature of the alloy, is lowered as a significant adverse effect.
As for the tensile strength, it was suggested to increase strength and improve strength stability by adding Y to Mg—Li alloys (Japanese Patent Publication No. 8-23057B), but using Y, which is an active element similarly to Li, naturally failed to solve the problems associated with corrosion resistance.
Furthermore, the increase in tensile strength in alloys obtained by adding Ag to Mg—Li eutectic alloys has also been reported, but using expensive material such as Ag is undesirable because of increased production cost of the alloys.